The present invention relates to lasers, and, in particular, relates to free electron lasers, and, in greater particularity, relates to wigglers used therein to modulate the free electron beam flowing therethrough.
In a free-electron laser (FEL), an electron beam is passed through a periodic magnetic field created by a device called a wiggler. The interaction with the magnetic field induces a transverse velocity modulation on the electron beam. The velocity modulation interacts with a radiation field to produce a ponderomotive force which drives a density modulation on the electron beam which can produce radiation which is in phase with the initial radiation field. The wavelength of the radiation thus generated is proportional to the wiggler period and inversely proportional to the square of the relativistic factor. The wiggler is a critical element in the FEL design.
Present magnetic wigglers/undulators use a configuration of permanent magnets or current carrying coils to produce the desired periodic spatial variation in the magnetic fields. Conventional wigglers typically have periods on the order of centimeters or tenths of centimeters. Furthermore, the field strength of the wiggler amplitude is severely limited as the wiggler period decreases because of constraints imposed by the materials and the configuration. It is difficult to vary the period of conventional wigglers because this necessitates reconstruction of the wiggler. Conventional wigglers, however, can have the amplitude of the magnetic field variation changed readily since, for those wigglers constructed using a number of current carrying coils, the coil current can be individually controlled.
Examples of prior art wigglers are shown in U.S. Pat. Nos. 4,893,103; 4,864,575; 4,679,197; 4,599,724; 4,542,510; and 4,438,513. For example, in U.S. Pat. No. 4,893,103 a plurality of magnetic rings are formed having opposing field direction from one ring to the next. Superconducting sheets are placed on these rings to modify the fields therein. The electron beam is sent down the middle of the rings and interacts with the periodic field established. The period of this field can not be changed without major modification to the wiggler structure. In U.S. Pat. No. 4,864,575 a periodic magnetic or electric field is established by a plurality of conducting layers separated by insulating layers and appropriately connected to provide either an electric or magnetic field. The electron beam is sent down a hole running perpendicular to the sheets. Multiple holes are provided. Because of the thin sheets, micron wavelength radiation is possible therefrom. The wavelength is not variable without major structural change. U.S. Pat. No. 4,438,513 provides a tunable radiation source without further detailing of the magnetic field.
Therefore there exists a need for a wiggler having a variable short period magnetic field structure.